Melt the butter. Mix flour and cocoa with baking soda and salt in a bowl. Stir together both types of sugar with the melted butter. Add egg yolk and egg, and beat until it's pale and fluffy. Stir in the flour mixture, and finally chocolate and nuts.

Scoop onto baking sheets, making sure to leave enough room to let the cookies spread - and top each with three mini marshmallows.